Hydrogen gas and metal-air batteries exhibit many advantages as a carbon-free energy storage medium. Both have the highest energy density by mass of all energy storage technologies. The hydrogen gas may be readily formed in a sustainable fashion via the electrolysis of water powered by renewable energy, and metal-air batteries may also be charged by an application of renewable electricity. Widespread adoption of hydrogen fuels and metalair batteries depends on control over electron transfer reactions, in particular the oxygen evolution reaction (OER), written as:2H2O→O2+4H++4e−  (1)in the case of low pH water electrolysis, or written as:4OH−→O2+2H2O+4e−  (2)in the case of high pH water electrolysis, or lastly as:MxOy→(y/2)O2+xM(2y/x)++4e−  (3)in the case of metal-air batteries, all of which occur at catalyst surfaces. For (1) and (2), the OER occurs in solar-driven water splitting or an electrolyzer, using electricity to produce hydrogen and oxygen gases. For (3), the OER occurs in an electricity-driven charging of metal-air batteries, using electrical power to form a reduced form of metal and/or metal oxides, and oxygen gases.
Well-known catalysts for the OER include expensive precious metals and precious metal oxides, for example IrO2. However, the prohibitive cost and scarcity of precious metal elements limit their usage in practical applications. First-row transition-metal oxides (such as NiCo2O4 and cobalt-phosphate-based catalysts) offer alternative solutions, but can be less active than IrO2.